Rocket nozzles are an elegant example of surfaces which are subjected to very hot, high velocity, erosive gases, and sometimes also to abrasive solid particles. Attempts have been made to apply a refractory coating to these nozzles, but these have generally depended on strictly physical forces to make the coating adhere to the substrate. Under these circumstances, cracking and spalling of the coating can and do occur. Similarly, other surfaces are exposed to similar stringencies. Examples are such as turbine blades, heat shields and hypersonic structures. This invention relates to coatings suitable for these general types of surfaces, and to processes for making them.
Further to confound the matter, these coatings have to be applied quite thickly to assure that the necessary physical retention forces are provided--often over 0.050 inches thick. But as a consequence, thermal stresses can be set up during the very rapid heating that occurs in the first few seconds after application of these stringencies, as well as erosive and thermal stringencies which persist during continuing operation. These stringencies often exceed the coating's capacity to withstand, and cracking and spalling can occur. It is even possible, with prior art coatings, for them to develop micro-cracks during processing, which later become regions of weakness.
When the coating cracks or spalls, the hot, high velocity gases are able to get underneath the remaining coating material and remove it, thereby exposing the areas they are intended to protect. In blade and structural applications the velocity of the body itself relative to the gases adds to the problem. This is, of course, intolerable, so the art has developed away from refractory coatings, and has utilized instead such expedients as metal structures whose surfaces can withstand these conditions. An example is the use of wrought tungsten inserts, which are excessively heavy, and are costly.
Because the bond between the prior art coating and the substrate is quite weak, the coating adds little to the strength of the structure, especially at the nozzle insert in rocket nozzle applications. For this reason, expensive high density pyrolytic graphite is often used, even in less stringent applications.
It is the object of this invention to provide a refractory coating which can be applied in thin sections so as to minimize internal thermal stresses, and yet still be so adherent to the substrate that is unlikely to crack or spall. It is very hard, and has a high melting point so that erosion of the coating is substantially eliminated and is strongly bonded to the substrate so that its strength is useful to the structure, and less expensive substrates can be utilized.